1. Field of the Invention
This invention relates to liquid crystal devices and more particularly, to a memory or storage cell embodying a liquid crystal medium.
2. Background Information
Great interest has focused during the past ten years or so on liquid crystal displays used in calculators, digital watches, small screen television sets and like equipment.
A variety of liquid crystal materials have been studied and these materials exhibit phases such as the nematic, the smectic, the cholesteric, as well as other phases. Within the phase classifications, a number of different molecular alignments can occur, depending on temperature, electric field and other parameters. One of these alignments is called homogeneous, which refers to an alignment that is parallel to the plates which are utilized in formation of the cell; whereas a homeotropic alignment occurs when the molecules are perpendicular to both of the plates. Yet another designation is the focal conic texture, which is characterized by a disordered or scattered orientation. Each of the foregoing, that is, the homogeneous, homeotropic or focal conic is referred to hereinafter as a state of the liquid crystal material and, hence, of the cell of which it forms a part.
As will be appreciated, by referring, for example, to U.S. Pat. No. 4,291,948, liquid crystal display cells that can provide a variety of color states and which can operate in a storage mode, can be formed of material that has positive or negative dielectric anisotropy. A smectic liquid crystal medium is discussed in that patent, such medium being disposed between electroded plates that are treated to produce parallel, that is, homogeneous alignment with a very large tilt angle. Variation of this tilt angle, as described in that patent, can be progressively increased by increasing the strength of an applied alternating potential, thereby producing Newtonian colors when viewed through crossed polarizers.
In accordance with one embodiment described in U.S. Pat. No. 4,291,948, a storage mode is achieved by variation of the aforenoted tilt angle. That is to say, when the tilt is increased from an initial value, it turns out that this increase is sustained after removal of the exciting field which caused it. However, as explained in the patent, the cell is switched back to the original lower limit of tilt angle by a heating and cooling cycle, whereby the material goes through its nematic phase and back to the smectic phase. In this connection, see FIG. 4 of this patent. Summarizing the operation of a typical cell as disclosed in U.S. Pat. No. 4,291,948, the cell is capable of being switched in only one direction by the application of an alternating electric potential, while thermal cycling is used for switching in the opposite direction. Moreover, as noted above, any storage that is achieved is based on variation of the tilt angle responsive to application of an exciting field. The only exception to this is the suggestion contained in column 4 of U.S. Pat. No. 4,291,948 to the effect that certain smectic phase materials have the property of exhibiting a cross-over frequency effect in which the material exhibits positive dielectric anisotropy at low frequencies below the crossover frequency and a negative dielectric anisotropy at high frequencies above the crossover frequency. The patentee further states that with such materials, electric switching in both directions is possible. However, except for this, any explicit teaching of bistability depends upon a heat treatment to change the stable state of the liquid crystal cell. In addition, such liquid crystal cell in U.S. Pat. No. 4,291,948 involves simply the application of a uniform field to the liquid crystal medium embedded between the two electrodes or plates of the cell.
Other references that also disclose the concept of storage in one form or another in a liquid crystal display are the following: U.S. Pat. Nos. 3,703,331 and 4,105,288. The former patent discloses a liquid crystal element which is constituted of a chiral nematic (or cholesteric) phase liquid crystal material. The latter patent refers to liquid crystal materials which demonstrate a quasi-bistability effect under certain boundary conditions. That is to say, when the applied electric field sees a certain threshold value, the material passes out of the energetically stable (focal conical) structure into a homeotropic-nematic orientation; and this orientation is maintained until the electric field is reduced below a second threshold value, with a brief formation of a planar-conical intermediate state.
Another reference of interest is an article by R. A. Soref entitled "Field Effects in the Nematic Liquid Crystals Obtained with Interdigital Electrodes", Journal of Applied Physics, Vol. 45, No. 12, December 1974. This article discloses certain interesting field effects in nematic liquid crystals. However, the device disclosed is provided with only two electrodes and does not provide permanent or stable state storage. In other words, refresh circuitry would have to be provided in order to renew the charge on any inherent or separate capacitance that might be involved with the basic device structure.
Whatever the advantages, and features of the aforenoted references, none of them discloses the features and advantages of the present invention.
Accordingly, it is a primary object of the present invention to provide a storage or memory device that is based upon the use of a liquid crystal material, preferably of the smectic phase, but depends on selective application of voltage pulses to discrete electrodes to effectuate stable states.